Systems for transporting cargo in transferable containers, carried on flatbed trailers towed by a tractor, are becoming increasingly popular. Such containers, for example, are particularly useful for "partial loads" where two or more containers may be carried on the same trailer destined for different destinations. To further increase the utilization of container/trailer transportation systems, containers have been designed with rollers or wheels on the bottom to facilitate loading and unloading of the containers from a trailer.
Three patents which disclose container systems permitting the transfer of container loads to and from either end of a semi-trailer onto a loading dock or onto another truck are the U.S. patents to Ault U.S. Pat. No. 2,631,885; Black U.S. Pat. No. 2,986,408; and Cayton U.S. Pat. No. 4,203,697. The Ault patent is directed to a container transfer system which uses pivot rails on the trailer to permit loading and unloading of pallets from either side of the trailer, or from either end. The Black patent is directed to a reversible semi-trailer construction for carrying two containers, which may be rolled onto and removed from either end of the trailer.
The patent to Cayton is directed to a trailer which is to be parked near an agricultural field. Pallets containing agricultural products are loaded onto a small transfer truck, which then backs up to either end of the trailer and rolls the pallets onto the trailer. The trailer may be made of a greater length than the length of the bed of the transfer vehicle; so that multiple pallets may be transferred from the smaller vehicle onto the trailer.
Two other patents directed to plural unit trailers having somewhat complex configurations are Bennett U.S. Pat. No. 3,163,306 and McCleary U.S. No. 3,504,814. The devices of both of these patents permit loading and unloading from either end of the trailer.
Another industry which has been unrelated to container transportation systems is the hot mix asphalt (HMA) industry. This industry is composed of several entities. The heavy tar-like residue, which remains after the production of gasoline and diesel fuel, is marketed in several grades and sold as what is called "bitumen" to hot mix asphalt producers (HMAP). The HMAP store the bitumen in large storage tanks and maintain the temperature of the bitumen at approximately 300.degree. to keep it in a liquid state. Whenever bitumen is allowed to cool, it solidifies and is not workable.
The producer has a hot mix plant (HMP) which includes aggregate storage facilities, conveyors, aggregate dryer drums, aggregate use bins with weighing devices to proportion different aggregate gradations, and the like. The producer selects the desired HMA formula and sends the desired proportions to a heated drum and dries the aggregate to the desired moisture content. Pugmills or other devices are used to mix the hot aggregate with asphalt cement (a binder) and bitumen into a well blended, homogenous mass. The product is called hot mix asphalt (HMA); and the producer markets HMA in several different formulas or grades.
Asphaltic cement, the binder in HMA, is a thermoplastic material with a characteristic of being very workable at 300.degree. Fahrenheit and as hard as concrete at 100.degree. Fahrenheit. Clearly, the workability of this material varies with temperature. At the job site, the placing crew typically likes to have the HMA material at close to 300.degree. Fahrenheit for workable paving. A typical HMA production facility, however, does not like to exceed 310.degree. to 320.degree. Fahrenheit, because light end hydrocarbons in the form of smoke and vapors become abundant at these higher temperatures. In addition, the combination of high temperatures and exposure to oxygen causes oxidation (hardening) or rapid aging of the asphaltic cement. Hot mix asphalt is a flexible pavement; and as long as it remains so, it is serviceable. As asphaltic pavement ages or hardens, it becomes brittle, cracks, and ultimately no longer is serviceable as a pavement structure. Consequently, any premature aging or hardening (oxidation) of asphaltic cement results in shorter service life of the asphalt pavements.
At HMA production facilities, care is given to protecting the HMA from rapid high temperature aging. Storage facilities now are available which are heated, insulated, sealed airtight, and in some cases, held under slight pressure from an inert gas generator, to avoid any oxygen exposure to the HMA as it is being stored. Storage facilities of this type have been approved by various government agencies to store HMA for several days. Typically, however, HMA is stored in large storage tanks constructed in this manner at the production facility, and then is transferred from the tanks to the dump trucks for transportation to the job site.
The different HMA products are stored in separate storage bins. Contractors order the formula they desire. Typically, hot HMA is weighed into dump truck and the HMA is delivered to the construction site. At the construction site, the HMA is transferred or dumped from the delivery truck into a paver, which spreads the HMA to the desired width and thickness. The HMA then is compacted to a dense mass by using heavy rollers. After the HMA cools to the ambient temperature, it becomes very hard and makes an excellent surface for highway pavements, parking lots, driveways, walking paths, and the like.
In cold climates, typically starting around Labor Day, it is difficult to place an HMA finish or top course, since the present methods of transporting and placing HMA allow the HMA to cool, where it becomes difficult to obtain an ideal paving surface. Since hot HMA cools relatively rapidly after it is placed in a delivery dump truck, the hot mix plant typically is located a relatively short distance from the application site. Presently, permanent hot mix asphalt plants economically service an area with a radius of approximately 25 miles. Beyond that radius, paving areas develop which may be considered "soft spots" because permanent hot mix asphalt plants typically are not located close enough together to cover all areas of the country. This has created a market for portable hot mix asphalt plants.
Whenever a contractor requires a large order of HMA (typically several thousand tons), a portable plant may be set up by the producer near the distribution site; so that unnecessary cooling of the HMA between the production site and the distribution site does not take place. This is done for major highway paving jobs, and the like. For some locations, however, which fall into the described "soft spots", zoning and aggregate availability limitations may preclude setting up a portable plant for even large jobs. In any event, customers who are located in the "soft areas" generally pay a higher rate for HMA, and frequently receive material which must be reheated at the distribution site, or which is in risk of being cooled to a temperature where application becomes difficult.
Another disadvantage of the typical HMA production and distribution system is that the material must be produced and heated to its application temperature only a relatively short time prior to its application at the job site. Consequently, production at an HMA plant typically takes place in the early morning hours; and the dump trucks which distribute the HMA material from the plant to the job site travel back and forth between the plant and the job site. If heavily traveled highways exist between the plant and the job site, the transportation of HMA material from the plant to the job site and the return trip of the empty truck can take undue amounts of time. This is particularly true if delivery is desired during the typical "rush hour" traffic congestion, which takes place in and around most large cities in the morning and in the late afternoon.
The size of the HMA industry in the United States is approximately 400 million to 500 million tons of HMA annually. At present market prices, this constitutes a nearly twenty billion dollar annual industry. In the United States, approximately 75 percent to 85 percent of HMA is used in the construction of pavements in highways, streets and airports. The balance of the HMA produced is used in private work, civil projects, parking lots, driveways, etc. The foregoing discussion is directed primarily to the production and delivery of HMA to the large construction projects constituting highways, streets and airports. Small jobs are difficult to effectively service; and many of them are located in the "soft spots". Because of the small size of such jobs, it is not practical to set up a portable HMA plant at the job site. Consequently, the delivery and distribution of HMA to many small job sites is inefficient; and the quality of the delivered HMA frequently is less than desired.
Typically, HMA is transported to paving sites in open dump truck bodies. During cold weather, truck boxes for transporting HMA are insulated to the extent possible, and frequently are covered with a roll-out tarpaulin. As an effort to maintain the HMA at a desired temperature, a system disclosed in the U.S. patent to O'Brien U.S. Pat. No. 5,120,217 has been developed. This patent discloses a delivery unit for HMA material. The delivery unit is mounted on a trailer. The hopper of the unit has hydraulically operated doors for closing its top; and it incorporates a double walled construction, with the inner hoppers spaced from the outer walls of the container. Heated air is passed in the space between the inner and outer hoppers, and also through a double wall in the doors in the top, to exit over the material inside the hopper. This unit is designed to be dropped off at a construction site at the end of a working day. A heater then is operated overnight; so that the material is at the desired temperature by the next morning and is then ready for use.
Another effort to provide heating for the hopper or truck used to carry HMA is disclosed in the U.S. patent to Eldridge U.S. Pat. No. 4,978,068. This patent discloses the passage of exhaust gases of a vehicle between inner and outer chambers of a hopper for heating the contents of the hopper. A different approach for heating the contents of a hopper carried on a truck is disclosed in the U.S. patent to King U.S. Pat. No. 4,695,186. The hopper disclosed in this patent is permanently mounted on a truck. The hopper structure includes a space between the lower walls of the hopper and an additional plate in the truck. This space provides a chamber for a heat exchange from a heater located beneath the hopper to transfer heat to the material within the hopper.
The U.S. patent to Dillingham U.S. Pat. No. 4,944,632 also discloses the use of a double wall construction in a hopper for transporting asphalt repair materials from one place to another. The hopper is open at the top; but the heat source is located to supply heat to the space between the walls for heating the asphalt in the hopper.
Accordingly, it is desirable to provide an improved distribution system, particularly suited for the delivery and distribution of HMA, which overcomes the disadvantages of the prior art, which is simple and effective to execute, and which is reliable in operation.